1. Field of the Invention
The present invention relates to an antiferroelectric liquid crystal material and its fabrication, more particularly to a chiral swallow-tailed liquid crystal material and its fabrication.
2. Description of the Prior Art
Currently, the antiferroelectric phase discovered in chiral liquid crystal has a tristable switching property, i.e., in addition to the bistable property of regular ferroelectric liquid crystal, chiral liquid crystal has an antiferroelectric chiral smectic C phase property, hereinafter called SCA*, or the so-called xe2x80x9cthird stable statexe2x80x9d.
Normally, antiferroelectric liquid crystal and ferroelectric liquid crystal have a similar helical structure. The helical structure of antiferroelectric liquid crystal is composed of zigzag bilayers in which, as shown in FIG. 1, the liquid crystal molecules of each two adjacent layers are arranged in reversed directions. In the helical structure of ferroelectricl liquid crystal, the liquid crystal molecules of each two adjacent layers are arranged in a same direction, as shown in FIG. 2.
The helical structure of antiferroelectric liquid crystal can be surface stablized to the state like ferroelectric liquid crystal by means of the unwound effect between liquid crystal molecules and interface. Under this surface stabilized state, due to that the liquid crystal molecules are arranged in a same direction, the ferroelectricl liquid crystal shows a same direction of molecule dipole as shown in FIG. 3. This feature causes the regular ferroelectric liquid crystal to produce pure spontaneous polarization. On the contrary, the liquid crystal molecules of the antiferroelectric liquid crystal show a zigzag bilayer structure under the surface stabilized state, as shown in FIG. 4, and the dipole of the molecules is respectively set off, without causing pure spontaneous polarization. This state of reverse molecular arrangement is the third stable state, which can be switched to the ferroelectric state by means of the application of an electric field. This switching is the so-called xe2x80x9cfield-induced antiferroelectric to ferroelectric switchingxe2x80x9d. Under the effect of an electric field, the tristable state of antiferroelectric shows particular electric-optical effects, for example, the properties of DC critical electric field and hysteresis. Theses properties can be used to improve design limitations on LCD (liquid crystal display) viewing angle and contrast ratio.
Further, the antiferroelectric liquid crystal provides the following advantages:
(1) The optical axes of the molecules of antiferroelectric liquid crystal molecules are arranged along the alignment, which facilitates to alignment stability;
(2) Under the effect of unwound, the antiferroelectric liquid crystal has the third stable state, that effectively eliminates ghost effect and memory effect;
(3) Under the effect of an electric field, the antiferroelectric liquid crystal has DC critical electrical field and retarding properties, therefore it increases matrix addressing capacity and improves LCD resolution;
(4) The antiferroelectric liquid crystal tends to obtain quasi bookshelf alignment structure, therefore it enables the LCD to have high value of contrast ratio (about 20xcx9c30);
(5) The antiferroelectric liquid crystal has a rapid response time, which enables current LCD driving technique to be fully utilized, and therefore it is not necessary to develop a new driving technique; and
(6) The antiferroelectric liquid crystal has a self-alignment recovery property; therefore it greatly improves LCD""s mechanical resisting and heat impact resisting capability.
As indicated above, the antiferroelectric liquid crystal plays an important role in the manufacturing and application of photoelectric apparatus. Photoelectric apparatus manufacturers and research units pay much attention to the study of the molecular structures of antiferroelectric liquid crystal materials and the relationship between liquid crystals, so as to design a low-cost, high-performance antiferroelectric liquid crystal material for making LCDs.
The molecular structure of currently developed antiferroelectric liquid crystal materials is similar to ferroelectric liquid crystal molecules. Referring to FIG. 5, both are commonly composed of a terminal chiral alkyl chain 1, a rigid core 2, a linking group 3 and an achiral alkyl chain 4. The molecular structure of the terminal chiral alkyl chain, the rigid core and the linking group is the key factor for the formation of antiferroelectric liquid crystals.
The terminal chiral alkyl chain structure in the molecular structure of the antiferroelectric liquid crystal has four different kinds as shown in FIG. 6. The polarity of molecular size of the substituent (R1) of the chiral center C* is the main factor that affects the formation of antiferroelectric liquid crystals. The structure change of the rigid core has little effect to the formation of antiferroelectric liquid crystals. In materials having different structures of rigid core, as shown in FIG. 7, the change of the rigid core structure from an aromatic ring to an iso-aromatic ring or the one having a substituent does not affect the formation of antiferroelectric liquid crystals. Further, most rigid cores are composed of at least three aromatic rings or iso-aromatic rings. Few antiferroelectric liquid crystal materials have a two-ring structure.
The linking group in an antiferroelectric liquid crystal molecular structure is normally of an ester group or ketone group. As illustrated in FIG. 8, the structure, which is linked between the rigid core and the terminal chiral alkyl chain, is most important. In a recent study on the radiation of X-rays and FTIR (Fourier transform infrared ray) spectrum to ester materials, it is reported that chiral tail linking ester group, which is linked by xe2x80x94COOxe2x80x94, may produce a bent structure, causing the molecules at two adjacent layers to form a reverse pair arrangement of zigzag bilayer structure. This xe2x80x94COOxe2x80x94linking group increases conjugation of the internal electrons of liquid crystal molecules in molecular long axis, that facilitates the formation of antiferroelectric SCA* liquid crystals.
Because the antiferroelectric liquid crystal has tristable switching property, DC critical electric field property (Eth) and retarding property, it is an appreciated key material for making high quality LCDs. However, due to the constraint of the influence of high Eth value and pretransitional effect, conventional antiferroelectric liquid crystal materials do not provide broad viewing angle and high contrast ratio as expected, when used in the fabrication of LCDs.
Till 1996, Japanese scientist Inui mixed the three antiferroelectric liquid crystal materials (I,II, and III) indicated in FIG. 9 at different ratios, and made a study to see the result of different mixing ratios on Eth value and pretransitional effect. This study shows that changing the mixing ratio of these liquid crystal materials effectively reduces antiferroelectric liquid crystal""s Eth value, however the change of the mixing ratio causes pretransitional effect more significant. According to Inui""s report, when the mixing ratio of I:II:III=40:40:20, no Eth value is found, and its field-induced antiferroelectric to ferroelectric switching shows a V-shaped switching. Inui gives the name of xe2x80x9cThresholdless antiferroelectric liquid crystalsxe2x80x9d to this antiferroelectric liquid crystal mixture. These thresholdless antiferroelectric liquid crystals have the following properties:
(1) great tilt angle ( greater than 35xc2x0);
(2) low driving voltage ( less than 2V/umxe2x88x921);
(3) ideal gray scale;
(4) fast antiferroelectric to ferroelectric switching time ( less than 50 xcexcs);
(5) broad viewing angle ( greater than 60xc2x0);
The aforesaid properties eliminate the gray scale problem occurred during the fabrication of a passive matrix addressing surface stable ferroelectric liquid crystal display, and also improve the drawback of being difficult to obtain a high contrast ratio commonly existed in regular active matrix or thin film transistor (TFT) addressing type deformed-helix ferroelectric liquid crystal displays and passive matrix addressing type antiferroelectric liquid crystal displays. In the meantime, the aforesaid properties can improve image qualities, particularly for gradational display, such as contrast, while retaining a high-speed responsiveness. It is possible for obtaining a liquid crystal display device, which can display gray scales with high speed and wide viewing angle.
In view of the numerous advantages of the aforesaid thresholdless antiferroelectric liquid crystals, it is one objective of the present invention to provide a chiral swallow-tailed liquid crystal, which is composed of the compound of (N,Nxe2x80x2-diethyl (s)-2-{6-[4-(4xe2x80x2-alkoxyphenyl)benzoyloxy]-2-naphthyl}propionamide or the compound of (N,Nxe2x80x2-dipropyl(s)-2-{6-[4-(4xe2x80x2-alkoxyphenyl)benzoyloxy]-2-naphthyl}propionamide; the chiral center of the present chiral compound is directly linked to its rigid core, and the chiral tail of this compound is linked with a swallow-tailed group. The chiral center and swallow-tailed group are arranged at the same side in the molecular structure. This structural characteristic enables the chiral compound to have the same property of thresholdless antiferroelectricity, and the advantage of simple manufacturing process. The present chiral compound shows an optimum application effect in liquid crystal displays.
It is another objective of the present invention to provide a chiral swallow-tailed liquid crystal, which has optical-electrical properties such as antiferroelectricity and thresholdless V-shaped switching, advantageously optimizing the mixing ratio of the liquid crystals for the manufacture of V-shaped thin film transistor liquid crystal displays.
It is a further objective of the present invention to provide a chiral swallow-tailed liquid crystal, which can be used as the liquid crystal material for active matrix addressing type liquid crystal displays.
In order to achieve the above objectives, the present invention provides a chiral swallow-tailed liquid crystal, which is synthesized from a chiral material. The present chiral swallow-tailed liquid crystal is composed of the compound of (N,Nxe2x80x2-diethyl (s)-2-{6-[4-(4xe2x80x2-alkoxyphenyl)benzoyloxy]-2-naphthyl}propionamide or the compound of (N,Nxe2x80x2-dipropyl(s)-2 -{6-[4 -(4xe2x80x2-alkoxyphenyl) benzoyloxy]-2-naphthyl}propionamide. The chiral center of the present chiral compound is directly linked to its rigid core, and the chiral tail of this compound is linked with a swallow-tailed group. The chiral center and swallow-tailed group are arranged at the same side in the molecular structure. This structural characteristic enables the chiral compound to have the same property of thresholdless antiferroelectricity, and the advantage of simple manufacturing process. The present chiral compound shows an optimum application effect in liquid crystal displays.